A plate-like sipe blade is provided inside a tire vulcanization mold to form a sipe in a tread portion of a pneumatic tire. As is shown in FIG. 14A, through-holes 228 are drilled in a sipe blade 226 to let air and rubber pass through during vulcanization molding.
As is indicated by an arrow in FIG. 14B, rubber fills in the through-hole 228 during vulcanization molding, when the mold is opened to remove a tire after the rubber sets, as is shown in FIG. 14C, rubber 244 in the through-hole 228 may possibly be torn off and remain in the through-hole 228. In such a case, the rubber 244 may later fall into the mold and adhere to an unvulcanized tire before subjected to vulcanization molding, thereby making the tire defective. The rubber 244 remaining in the through-hole 228 raises a problem particularly in a case where the mold is a so-called two-piece mold formed of an upper mold and a lower mold because when the rubber 244 falls, it often stays on top of the lower mold.
In addition, cracking occurs in a sipe edge in some cases when rubber in a through-hole is torn off. A countermeasure proposed to eliminate this inconvenience is to provide a difference in diameter or a step to the through-hole (see, for example, JP-A-61-291205).